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1.4.1 Motifs and supersecondary structures

Supersecondary structures or motifs are particular arrangements and combinations of two or three secondary structures, often with defined topology (or connectivity). Table 3: view document describes some of the most common of these.

The term ‘motif’ is also used to describe a consensus sequence of amino acids, i.e. a partial sequence c
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1.3 Protein secondary structure

From our consideration of the steric constraints that apply to peptide bonds and amino acid residues in a polypeptide, we have already begun to discuss some of the factors that determine how the backbone of the polypeptide folds. The conformation adopted by the polypeptide backbone of a protein is referred to as secondary structure. Whilst it is true to say that all proteins have a unique three-dimensional structure or conformation, specified by the nature and sequence of their amino a
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1.1 So what's it all about?

iSpot is a website aimed at helping anyone identify anything in nature. Once you've registered, you can add an observation to the website and suggest an identification yourself or see if anyone else can identify it for you. You can also help others by adding an identification to an existing observation, which you may like to do as your knowledge grows. Your reputation on the site will grow as people agree with you identifications. You may also like to visit our forums which offer lively debat
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1.6.4 Drop-towers revisited

In Section 1 we described how research into near weightless conditions can be carried out on Earth by using a drop-tower or a drop-shaft (Figure 41). We are now in a position to examine drop-shafts in more detail (Example 3).

Figure 41
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1.5.5 Derived functions and derivative notation

Given the function x(t) that describes some particular motion, you could plot the corresponding position–time graph, measure its gradient at a variety of times to find the instantaneous velocity at those times and then plot the velocity–time graph. If you had some time left, you might go on to measure the gradient of the velocity–time graph at various times, and then plot the acceleration–time graph for the motion. This would effectively complete the description of the m
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1.5.4 Functions and the function notation

In Figure 25, the position x of the car depends on the time t. The graph associates a particular value of x with each value of t over the plotted range. In other circumstances we might know an equation that associates a value of x with each value of t, as in the case of the equation x = At + B that we discussed in Section 3. You can invent countless other ways in which x depends on t: for instance x = 
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1.5.3 A note on functions and derivatives

This subsection introduces two crucially important mathematical ideas, functions and derivatives, both of which are used throughout physics.


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1.5.1 Instantaneous velocity

Uniform motion is simple to describe, but is rarely achieved in practice. Most objects do not move at a precisely constant velocity. If you drop an apple it will fall downwards, but it will pick up speed as it does so (Figure 24), and if you drive along a straight road you are likely to encounter some traffic that will force you to vary your speed from time to time. For the most part, real motions are non-uniform motions.

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8.3 Shortage of minerals

You may be familiar with salt licks that are provided for domesticated cattle. In the wild, grass is also often low in minerals (e.g. it has almost no sodium and very little calcium), so grazers may have to go to extraordinary lengths to supplement their diet with additional minerals obtained from the most unlikely places. LoM gives some examples, but the most impressive activity takes place in the caves of Mount Elgon in Kenya [pp. 113–114]. You'll probably recall this spectacular footage
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5.3 Hindgut fermenters

The odd-toed ungulates (comprising the order Perissodactyla), the horses, tapirs and rhinoceroses, are hindgut fermenters, as are elephants. Update Table 2 with this information. These animals have a relatively simple, small undivided stomach, but this time an even larger caecum and colon where the microbes are housed and whe
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6.1 Basic isotropy

As we have said, the photons in the 3 K background have been practically free from interaction with anything since about 4 × 105 years after the instant of the big bang. The present angular distribution of the microwave radiation – the way in which it is spread across the sky – is therefore almost the same as it was then. The spectrum we find today depends on the temperatures at that time – for the intensity of the radiation in a particular region of the early Unive
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5.3 The redshift of the 3 K radiation

The temperature, T, of the radiation is proportional to the most probable photon energy, E, which as we have said is proportional to f, and hence inversely proportional to the wavelength λ. Thus,

According to Equation 1, we have for the redshift, z


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5.2 The origin of the 3 K radiation

In speaking of the radiation as having a cosmic origin, what do we have in mind? Essentially this:

In the violent conditions of the early evolution of the Universe, a stage was reached where the matter consisted of a plasma of electrons, protons, neutrons, and some light nuclei such as helium. There were no atoms as such for the simple reason that atoms would have been too fragile to withstand the violence of the collisions that were taking place at the temperature that then existed. As
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5.1 A second major discovery

In the introduction to this unit, we said that there were three pillars of evidence for the big bang. We now turn to the second. It rests on a discovery that ranks in importance with that of Hubble's law. It came about when observations in a new region of the electromagnetic spectrum – the microwave region – became possible. This was due to the invention of new detectors, working at frequencies as high as 30 000 MHz. In 1965, two Bell Telephone scientists, A. Penzias and R. Wilson, were i
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4.2 Evidence for a big bang

Having interpreted the redshift as indicating a recessional speed proportional to distance, one may extrapolate into the future to predict how the positions of the galaxies will evolve with time. One can also run the sequence backwards, so to speak, to discuss what their positions were in the past. Clearly, at former times the galaxies were closer to each other.

But not only that. Because of the proportional relationship between speed and distance (Equation 6), at a certain time in the
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4.1 Hubble's discoveries

In this section, we bring together two important features of galaxies – their redshifts and their distances.

This crucial development owes its origins to Edwin Hubble. His pioneering work in 1923 first led to the confirmation that certain of the fuzzy patches in the sky, loosely called ‘nebulae’, were in fact galaxies like our own.

3.3 Extending the distance scale

Having reviewed some of the properties of galaxies, we are now in a position to return to the question of how we are to develop further our methods of measuring distance.

The various steps taken in determining larger distances from known smaller ones are often called ‘rungs in the distance ladder’. The process of constructing a rung has been:

  1. Find a measurable quantity associated with a class of objects.

  2. Observe how the measura
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3.2 Some general properties of galaxies

Firstly, we note that galaxies tend to occur in clusters rather than singly. The mutual gravitational attraction of galaxies naturally tends to hold them on paths that remain close to each other. Typically a cluster contains tens or hundreds of galaxies. There are, however, large clusters with thousands of galaxies, and there are some solitary galaxies. Our own Galaxy is a member of a smallish cluster of about 36 galaxies called the Local Group (see Author(s): The Open University

3.1 First steps towards a distance scale

As you will see from Table 2, when it comes to astronomy and cosmology, one is called on to deal with a wide range of distances. (Note that a light-year (ly) is the distance light travels in one year, i.e. 9.46 × 1015 m. The distances are also quoted in a very commonly used astronomical unit of distance: the megapar
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2 Radiation from the galaxies

Stars occur in great collections called galaxies. The distribution and motion of galaxies provide us with the first important experimental information on which we shall build our understanding of the type of universe we inhabit. So, what do we know about galaxies?

All the stars that can be distinguished by the naked eye – a few thousand in number – belong to one galaxy: our own Milky Way Galaxy. Sometimes it is just written Galaxy, with a capital G, to distinguish it from all
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